1. Field of the Invention
The present invention relates to a method and an apparatus for driving a liquid crystal display device and, more particularly, to a method and an apparatus for driving a liquid crystal display device which incorporates switching elements each having a nonlinear current-voltage characteristic in a one-to-one correspondence with pixels.
2. Description of the Related Art
Recently, a liquid crystal display device is used not only as a comparatively simple display device incorporated in, e.g., a timepiece, a portable calculator, or a measuring instrument, but also as a display device for displaying large-capacity information, e.g., a display device incorporated in a personal computer, a wordprocessor, an OA terminal station, or a TV image display. In such a large-capacity liquid crystal display device, a method of time-divisionally driving display elements, i.e., pixels arranged in a matrix manner is generally adopted. In this method, however, no sufficient contrast ratio can be obtained between a display portion constituted by pixels to be turned on and a non-display portion constituted by pixels to be turned off, due to essential properties of a liquid crystal itself. That is, the contrast ratio is degraded as scanning electrodes are increased and it is practically limited that the display device have about 200 scanning electrodes. The contrast ratio is significantly reduced in a large-scale matrix display device having 500 or more scanning electrodes. This reduction in contrast ratio is a fatal defect for a display device.
Systems for solving this problem of the liquid crystal display device have been widely developed in many places. In one system, individual pixels are directly switched, and a thin-film transistor is adopted as a switching element. Although various types of materials such as cadmium selenide and tellurium have been conventionally proposed as a semiconductor for forming this thin-film transistor, amorphous silicon is most widely studied recently. In the manufacture of a liquid crystal display device of this type, however, since a step of micropatterning must be performed a plurality of times, the manufacturing steps are complicated to lead to a poor yield. As a result, the product cost is increased, and it is very difficult to manufacture a large-scale liquid crystal display device.
As another system using a switching element array, a liquid crystal display device using switching elements (to be referred to as nonlinear resistive elements hereinafter) each having a nonlinear current-voltage characteristic is available. This nonlinear resistive element basically has two terminals whereas the number of terminals of the thin-film transistor is three. Therefore, the nonlinear resistive element has a simpler structure and can be easily manufactured. For this reason, since an improvement in product yield can be expected, the cost can be advantageously reduced.
As the nonlinear resistive element, a junction diode type using a material similar to that of the thin-film transistor, a varistor type using zinc oxide, a metal-insulator-metal (MIM) type in which an insulator is sandwiched between electrodes, and a metalx semi-insulator (MSI) type in which a semi-insulator layer is sandwiched between metal electrodes have already been developed. Of these types, the MIM type is one of those having the simplest structure and has already been put into practical use presently.
FIG. 1 shows a voltage waveform applied to a liquid crystal layer of the MIM type liquid crystal display device, in which the ordinate represents a voltage VLC applied to the liquid crystal layer and the abscissa represents time. In this MIM liquid crystal display device, when a drive voltage is applied to each pixel, the liquid crystal is charged at a small time constant. When application of the drive voltage is stopped, the liquid crystal is discharged at a large time constant. Therefore, as shown in FIG. 1, the liquid crystal is charged within a short select period ron from the ON timing of the drive voltage, and a sufficient voltage is held between the electrodes sandwiching a liquid crystal for a long period .tau.off even after the drive voltage is cut off. As a result, the application voltage during the select period .tau.on determines an effective value of the drive voltage. In the MIM type liquid crystal display device, therefore, an effective value ratio of an effective drive voltage during a period in which liquid crystal display elements transmit light with respect to that during a period in which these elements shut light can be increased to be higher than that obtained when a conventional matrix type display device is time-divisionally driven. Therefore, a liquid crystal display device which does not reduce the contrast ratio is realized.
In the MIM type liquid crystal display device as described above, since a current-voltage characteristic of each MIM element is not symmetrical in the positive and negative directions, a display screen flickers. In addition, when one display pattern is displayed over a long time period, the display pattern slightly remains for a while, i.e., an afterimage phenomenon occurs. The flicker can be suppressed by superposing a DC offset voltage on a drive waveform. The afterimage phenomenon, however, occurs even when the DC offset voltage is applied to suppress the flicker. When the ON/OFF effective value ratio is sufficiently high, i.e., when a liquid crystal display device having about 100 to about 300 scanning electrodes is time-divisionally driven, the afterimage phenomenon is so subtle as to be apparently negligible. However, when the ON/OFF effective value ratio is inevitably reduced, e.g., when a liquid crystal display device having about 300 to about 1,000 scanning electrodes is time-divisionally driven, the afterimage phenomenon is apparently enhanced. This afterimage phenomenon is a serious problem in practical applications because it significantly deteriorates display quality.